FET Flashcards
How to tell FET drain from source
drain is at positive voltage relative to source by convention
What is FET cutoff
Vgs=0, DS is back to back diodes
What is enhancement mode FET
normally off, need to control Vgs to turn on
What is depletion mode FET
normally on, need to control Vgs to turn off. used as loads in NMOS only process in old CPUs, Motorola 6800 and 6502
Describe NMOS
N-type source and drain. P-type substrate. The channel is created by inverting the p-type substrate to n-type with a value of Vgs that is high enough. The channel is also called an inversion layer.
How much charge is in the channel
Q=Cox(WL)Vov
How to solve for Ids in MOS
(Q/L)(drift velocity)=(Q/m)(m/s)=Q/s=A=(CWVov)(uVds/L) -> Ids = uC*(W/L)(Vov)(Vds) w/o pinch off
Note: Cox = how much charge. un=how fast
Regions of MOS operation
Off: Vgs < Vt
Triode: Vds < Vov, Id=f(Vds)
Saturation: Vds > Vov, Id=f(Vov) (pinch-off)
How to make PMOS in p-type subtrate?
Need an n-well
Best bias for linear NMOS
Saturate so it’s a VCCS with one input.
1) Vgs>Vt,
2) Vds>Vov,
3) Vgd
Best bias for linear BJT NPN
NPN in active mode (BC reverse biased)
1) Vbe(on)
2) Vbc < 0.4V
3) Vce > 0.3V
what is channel length modulation
As Vdd increases the channel moved away from drain. Since Id is inverse proportional to channel length, as it shortens Id increases with Vds. Lamba is the device parameter. Id=f(Vov)(1+LVds) or (1+Vds/Va)
what is the early voltage
specs channel length modulation. output impedance of FET
ro=Va/Id
What is the body effect
Changes in Vsb are reflected as a change in Vt
How does Vt change with temperature
down by 2mV for every degree C
